One typical automotive-type solid electrolyte exhaust gas oxygen sensor is disclosed in U.S. Pat. No. 3,844,920 Burgett et al. In this typical structure the solid electrolyte is a zirconia body shaped as a tapered thimble. The interior and exterior of the thimble have separate porous electrode coatings of platinum or the like. The inner electrode is exposed to air for establishing reference potential. The outer electrode is exposed to the exhaust gas for establishing a potential determined by its oxygen concentration. In the past, the inner electrode covered the entire inner surface of the thimble. An improved configuration for the inner electrode is shown in United States patent application Ser. No. 961,137 entitled "Heated Solid Electrolyte Oxygen Sensor", filed Nov. 16, 1978, in the names of Gamdur S. Mann et al, now U.S. Pat. No. 4,169,778, and assigned to the assignee of this invention. In this improved configuration, the inner electrode covers only the bottom of the thimble. An electrode stripe extends up the side wall of the thimble to its top edge to facilitate making electrical contact with the electrode.
The inner electrode has heretofore been formed by painting on a coating of the platinum ink onto the inner surface of the zirconia thimble. Because of the small size of these thimbles, it has been difficult to brush the platinum ink onto their inner surfaces. In fact, it has been the practice of at least one manufacturer to apply this coating by means of pipe cleaners or the like. In any event, each zirconia thimble was individually handled and painted. Sometimes two coatings were required, which doubles the risk of contamination, damage, cost, etc. Also, hand painting produced electrodes that had properties varying from part to part, due to inconsistencies in thickness area, configuration, and other like variations. As a result, yields of acceptable parts were not very high, even though tolerances were set rather wide. I have found how to apply the electrode ink consistently, controllably and rapidly without human handling during the coating process. I have found how to automatically selectively coat the thimbles even though there is little room within their interior. Still further, I have been able to apply a highly uniform coating that is smooth and predictable in thickness and electrical resistance. High reproducibility in both physical and electrical characteristics are obtained in a single coating step that requires no human handling. Consequently, yields of acceptable products can be increased. It even appears that an improvement in sensor performance is achieved.